Such construction elements may, for example, have the form of sheets or boards. They may be employed in particular in the field of concrete formwork. If they have the form of sheets, they have a thickness of about 12-23 mm, a length in the range of around 60 cm to 6 m and a width of around 20 to 250 cm. These sizes vary from manufacturer to manufacturer. The dimensional details give an idea of in which size range the construction elements are used if they are sheets. Broadly speaking, it may be said that length and width vary in order of magnitude in the lower meter range or decimeter range.
If the construction elements are boards, in principle the dimensions are in the same range. Such boards have thicknesses in the centimeter range, lengths in the lower meter range and widths in the lower decimeter range. Boards are used in the construction industry, for example for timber formwork girders, for example H girders. However, they may also be fixed as textured boards in front of the formwork frame. However, they may also be used as boards which make the bottom boards or the side boards on console balustrades.
Sheets are virtually always made up of a plurality of layers such as plywood and provided with a protective film. What is said below with regard to composite formwork sheets generally applies analogously to very many areas for plywood sheets:
1. The sheet expands differently in different directions when heated.
2. If film laminated sheets are subjected to high temperature loads, blistering and detaching effects occur.
3. It is difficult to make the surface repellent to cement paste. The surface loses these repelling properties over time, which is why separate cleaning installations and cleaning machines are necessary.
4. Release agents have to be used.
5. The use of vibrators may cause surface destruction, either by the vibrator touching the sheet directly, or else by the vibrator making the film layer undergo very short oscillations which cannot be absorbed by the bonding joint over long periods and therefore results in detachment. This risk exists in particular in the case of ceiling formwork.
6. The sheets absorb moisture and therefore change their shape after installation.
7. Mechanical damage pierces the film and brings about a loss in load-bearing capacity.
8. The sheets may rot due to decay or fungal attack.
9. The abrasion resistance is not the same over the cross-section, since the material is not homogeneous.
10. Textured sheets have a short service life and are difficult to produce.
11. It is difficult to repair sizeable drill holes in or damage to the formwork skin.
12. The boards are only roughly classified according to load-bearing capacity and modulus of elasticity. They cannot be adequately matched in accordance with use.
13. Recycling is not possible.
14. It is not possible to fix the shape to within tenths of a millimeter.
15. Since the edges are interfaces, they have to be sealed.
16. An elastic joint has to be produced by means of silicone between the nose of the frame leg and the formwork skin.
17. Plastic cones have to be fitted as edge protection.
18. The screws which fix the sheet to the frame cannot be screwed in completely because allowance must be made for the swelling of the sheet. Consequently, the fixing screws initially protrude, which presents difficulties in transportation and impairs the concrete finish.
19. The surface cannot be easily regenerated.
20. The service life is not long enough.
21. The sheets have to be dip-impregnated.
22. Allowance has to be made for the fact that timber will only withstand a certain amount of pressure while the metal supports would allow much higher pressure.
23. Edges can split. In the case of boards in particular, the ends splay.
24. In principle it is impossible to use recycled materials for production.
25. The sheets and boards must be stored protected from the weather.
26. Production is complicated. Modern processes such as compression molding or extrusion are not possible.
27. If the construction element absorbs moisture, the load-bearing capacity fluctuates.
28. The sheets become disproportionately expensive with increasing size of their surface area. However, at the same time there has been a trend for some time to use large-area formwork elements.
29. When fitting the sheets in the frame, the longitudinal and transverse directions have to be considered, since they have different moduli of elasticity even in the case of plied sheets.
30. When fixing sheets having a very high load-bearing capacity, a hole for the screw has to be predrilled in order to prevent the film tearing.